An assignment is created when you tell Project that a resource will be working on a task, such as putting the resource Brian to work on a task called Develop Requirements. The best place to view information about assignments is in one of the Usage views. In the following figure you can see the Task Usage view (Views, Task Usage). In it there is a task called Develop Requirements, and the resource Brian is assigned to work 16 hours on it. The row where Brian's name appears is the row of this view that represents Brian's assignment to the task, which starts on 6/30 and ends on 7/1.
If you assign another resource to this task, you'll start to see how the relationship between a task and its assignments are a bit like the relationship between a summary task and its subtasks .
Here we've added a second resource (Steve) to the task. The combined assignment's Work values roll up to the overall Work value for the task, which is similar to how subtask Work values add up to the total Work value for a summary task. Notice that the dates do this as well, because Brian is assigned for only the first two days, and Steve for only the last two days.
The assignment is also the most common place for progress information to be captured. In the following figure we have added the Actual Work field to both the timephased area on the right and the table on the left.
The project manager has entered Actual Work values for Brian for the 30th and the 1st, and for Steve on the 1st. Again, you can see that the Actual Work values have rolled up to the task level, so that the Task Actual Work is the sum of the Actual Work fields of the assignments.
It is also important to remember that a task can have many resources assigned to it and a resource can be assigned to many tasks , but a resource can only have one assignment to any given task. Put another way, a resource cannot be assigned to a task twice.